This invention relates to image motion compensation and more particularly to a system for maintaining a fixed relationship between a target to be photographed and an image of the target as recorded on a photographic film by use of a lock-on technique.
The subject invention is used in a variety of applications where movement of an image to be recorded or movement of a camera used to record an image is unavoidable. One such application is in the area of aerial photography.
One of the most important requirements of aerial photography is the undistorted reproduction of a recorded image. Compliance with this requirement is made most difficult because of the very nature of aerial photography. When an aircraft with an aerial camera is airborne numerous factors which cause a distorting of the recorded image come into play. Among these factors are forward movement of the aircraft, sidewise movement of the aircraft caused by side winds, changes in aircraft altitude, and vibration of the aircraft. The conventional techniques for image stabilization address themselves to solving the problems raised by one or more of the image distortion factors.
A fast shutter speed is the most conventional way of freezing a recorded image. This means has proven to be ineffective when applied to aerial photography. Since the aerial camera moves while the shutter is open, a blur is always produced. In addition a fast shutter speed necessitates the use of wide lens apertures and fast photographic emulsions which result in an unacceptable loss in image resolution. In the alternative the lens may be swung or the whole camera panned to follow the target. This method cannot be applied to aerial photography because of the weight, mounting restrictions and complexity of the aerial camera and associated apparatus.
The most widely accepted technique for effectively stopping the movement of an image relative to a film caused by the object or the camera moving during exposure is commonly known as image motion compensation. In image motion compensation a relatively sharp image of a moving object is recorded on film by advancing the film in step with the image movement or by moving the optical system.
Image motion compensation (hereinafter referred to as IMC) has been accomplished in a number of ways. A film is moved during exposure in step with the image movement, either by moving the film plate or register glass or by advancing the film itself. Various means of moving the film are used. The film may be pulled by its edges. The film may be held by suction to a moving plate. The back of the camera may be moved.
The speed and direction of image motion are assessed and the film is made to travel at this speed, while the camera is aligned to match the direction. In this way, the image remains virtually motionless on the film.
IMC permits slower shutter speeds and therefore the use of slower, more highly corrected lenses and slower fine-grain film.
The focal length of the lens, aircraft height, and speed determine film movement. These are often monitored and fed in electronically.
Even with IMC certain errors still exist. These errors include aircraft drift due to side winds, tilt and height variations. Drift and tilt errors may be eliminated by rotating and giro-stabilized camera mounts. Height errors may be corrected by re-sealing in the enlarger.